1. Field of the Invention
The present invention relates to a MIS gate structure type HEMT device and particularly a normally-off type HEMT device.
2. Description of the Background Art
Since a nitride semiconductor has a high insulation breakdown electric field and a high saturated electron velocity, the nitride semiconductor has attracted attention as a semiconductor material for high frequency/high power devices of next generation. In particular, a layered structure of Group III nitride materials such as AlGaN/GaN and AlInN/GaN can store a high concentration of two-dimensional electron gas in a layered interface due to significant polarization effects (spontaneous polarization effect and piezo-polarization effect) peculiar to the nitride materials. Development of a high electron mobility transistor (HEMT) using this characteristic has been actively carried out. (see “Highly Reliable 250W GaN High Electron Mobility Transistor Power Amplifier” Japanese Journal of Applied Physics, Vol. 44, No. 7A, 2005, pp. 4896-4901 (article 1), and “Design and Demonstration of High Breakdown Voltage GaN High Electron Mobility Transistor (HEMT) Using Field Plate Structure for Power Electronics Applications” Japanese Journal of Applied Physics, Vol. 43, No. 4B, 2004, pp. 2239-2242 (article 2)).
In a transistor device to be used for power applications (power transistor), it is desired to simultaneously make high withstand voltage and low-on resistance satisfactory. Additionally, in order to provide fail safe and simplify and miniaturize an electric power circuit, it is also, in general, desired for the device to be a normally-off operation type device in which no main current flows unless gate voltage is applied, that is, a threshold voltage is positive.
Techniques for making a shift of the threshold voltage to a positive side by making an AlGaN barrier layer thin have already been known (see “Non-Recessed-Gate Enhancement-Model AlGaN/GaN High Electron Mobility Transistors with High RF Performance” Japanese Journal of Applied Physics, Vol. 43, No. 4B, 2004, pp. 2255-2258 (article 3), and article 2: “Enhancement-Model AlGaN/AlN/GaN High Electron Mobility Transistor with low On-state Resistance and High Breakdown Voltage” Japanese Journal of Applied Physics, Vol. 45, No. 44, 2006, pp. L1168-L1170 (article 4)).
For example, in the case where an n-channel transistor such as GaN type HEMT device is made to be a normally-off operation type, it is more desirable that no main current flows unless the gate voltage is 3 V or high but not near 0 V in terms of prevention of erroneous operation by outside noise. That is, the threshold voltage is desirably +3 V or more.
The above-mentioned article 1 and article 2 disclose only normally-on operation type and GaN type HEMT devices showing a low on-resistance. Naturally, there is no disclosure or suggestion of a normally-off operation type GaN type HEMT device having a low on-resistance and a threshold voltage of +3 V or more.
Further, in the case where HEMT devices are formed by techniques disclosed in the above-mentioned article 3 and article 4, the piezo-effect is suppressed due to thinning of an AlGaN barrier layer and therefore, there occurs a problem that sufficiently high two-dimensional electron gas concentration cannot be assured and a low on-resistance cannot be obtained. Furthermore, since a Schottky junction is employed for gate parts, gate voltage (no higher than +1.2 V) exceeding a Schottky barrier height cannot be applied and thus it results in a problem that a threshold voltage of +3 V or more, which is said to be a safe operation range of normally-off devices, cannot be attained.